1. Field of the Invention
The present invention relates to an angular rate sensor used for detecting e.g., video camera shake, the operation of a virtual reality apparatus, the direction in a car navigation system or the like.
2. Description of the Related Art
A so-called oscillation gyro type angular rate sensor has been widely available as consumer use. The osillation gyro type angular rate sensor detects angular rate by oscillating a rod-like oscillator at a predetermined resonance frequency and detecting Coriolis force generated by influence of angular rate with a piezoelectric element or the like.
For driving an oscillator in the angular rate sensor like this, a method using a separately-excited oscillation type driving circuit and one using a self-excited oscillation type driving circuit are available. However, the method using a separately-excited oscillation type driving circuit has a problem that when a difference is made between oscillation frequency and resonance frequency of an oscillator due to influence of temperature characteristics of the oscillator or the like, sensitivity for detecting Coriolis force rapidly decreases. Therefore, the method using a separately-excited oscillation type driving circuit has not been in practical use.
Consequently, the method using a self-excited oscillation type driving circuit in which the oscillator is incorporated in a loop of a phase-shift oscillator circuit is now widely used. Since the angular rate sensor using this method self-oscillates at the resonance frequency of oscillator, sensitivity thereof hardly changes due to influence of the temperature characteristics, thereby obtaining an angular rate output having stable sensitivity in a wide temperature range (refer, for example, to Jpn. Pat. Appln. Laid-Open Publication No. 2000-131077).
A conventional angular rate sensor shown in FIGS. 1A, 1B, and 1C includes a triangular prism-like oscillator 104 having a triangular prism-like constant elastic oscillator 100. The constant elastic oscillator 100 has first to third piezoelectric elements 101 to 103 attached to the side surfaces thereof, respectively. The first piezoelectric element 101 is composed of an electrode 101a and a piezoelectric material 101b. The second piezoelectric element 102 is composed of an electrode 102a and a piezoelectric material 102b. The third piezoelectric element 103 is composed of an electrode 103a and a piezoelectric material 103b. For example, the constant elastic oscillator 100 is a constant elastic metal oscillator.
The conventional angular rate sensor includes an amplifier 105 connected to the first piezoelectric element 101, a phase shifter 106 connected to the amplifier 105, a differential amplifier 107 connected to the second and third piezoelectric elements 102 and 103, a synchronous detector 108 connected to the differential amplifier 107, and a low-pass filter 109 connected to the synchronous detector 108. In the conventional angular rate sensor, the second and third piezoelectric elements 102 and 103 detect oscillation of the oscillator 104 for performing self-excited oscillation as well as Coriolis force generated in the oscillator 104.
The angular rate sensor using the triangular prism-like oscillator 104 has the highest sensitivity at the present time and therefore is currently mainstream. However, the angular rate sensor of this type has a complicated structure, which makes it difficult to produce high volume efficiency in manufacturing process. For example, in the above configuration, process of bonding piezoelectric elements to each of the triangular prism-like constant elastic oscillators is required, with the result that volume efficiency cannot be improved. Further, along with the miniaturization of the sensor, accuracy in a support mechanism or bonding accuracy of the piezoelectric element to the constant elastic metal oscillator has been increasingly demanded. In addition, influence of a bonding layer to the oscillator is increased. Therefore, manufacturing efficiency is lowered and manufacturing cost is significantly increased.
Another conventional angular rate sensor shown in FIGS. 2A, 2B and 2C includes an oscillator 117 having a columnar piezoelectric ceramic oscillator 110. The piezoelectric ceramic oscillator 110 has six electrodes 111 to 116 printed on a side surface thereof. The first to third electrodes 111 to 113 are independently formed. The fourth to sixth electrodes 114 to 116 are connected to the same ground potential. This angular rate sensor includes an amplifier 118 connected to the first electrode 111, a phase shifter 119 connected to the amplifier 118, an adder 120 connected to the phase shifter 119, a differential amplifier 121 connected to the second and third electrodes 112 and 113, a synchronous detector 122 connected to the differential amplifier 121, and a low-pass filter 123 connected to the synchronous detector 122. This angular rate sensor applies a voltage to the first electrode 111 to oscillate the oscillator 117, and detects Coriolis force generated in the oscillator 117 with the second and third electrodes 112 and 113.
In this conventional angular rate sensor, the electrodes 111 to 116 are printed on the oscillator 117 as described above. This eliminates the need to bond the piezoelectric elements to the oscillator 117 and makes the structure of the sensor relatively simple. However, in the case where the sensor size is reduced, it is difficult to produce an accurately configured piezoelectric ceramic oscillator 110 and, it is also difficult to print the electrodes onto the piezoelectric ceramic oscillator 110 with high accuracy.
That is, while this conventional angular rate sensor uses the columnar piezoelectric ceramic oscillator 110, it is difficult to manufacture, with high accuracy, the columnar piezoelectric ceramic oscillator 110 as compared to the triangular prism-like or quadratic prism-like oscillator. Further, it is not easy to print the electrodes onto the rounded surface of this angular rate sensor with high accuracy. As described above, the use of the columnar piezoelectric ceramic oscillator 110 makes it difficult to produce the angular rate sensor in large volume. Even though the mass-production has been realized, it is difficult to reduce manufacturing cost.
In addition to the conventional oscillation configuration of the angular rate sensor, unimorph-type and bimorph-type oscillators are widely used.
In the unimorph-type and bimorph-type oscillators, a combination of the same materials such as PZT (Lead Zirconate Titanate) are used to form the oscillator in general, in order to reduce manufacturing cost by common use of raw materials, to stabilize workability, to increase accuracy, and to reduce adversely effect due to influence of adhesion performance or thermal characteristics change.
However, since the thermal characteristics change in a piezoelectric material, such as PZT or the like, is non-linear, it is difficult to reduce temperature change in some characteristics in the case where the PZT is used as a substrate. Further, when detection sensitivity or detuning degree (frequency difference between lateral and vertical oscillations) that greatly contributes the detection sensitivity or frequency responsibility needs to be freely designed, the above factors are uniquely determined by section size of the oscillator and materials of the substrate, with the result that design freedom of a configuration of the oscillator tends to be limited.
Further, in the case of non-polarized state (mono-morph) in which the same PZTs are laminated and thereby the substrate does not have piezoelectricity, characteristics change in the resonance resistance obtained in the case where a single body of the oscillator is self-oscillated or in the sensitivity of an angular rate sensor using the same oscillator with respect to environmental temperature is large. This characteristics change tends to increase with higher temperatures. Thus, it is necessary to compensate the characteristics change by means of an electric circuit in order to maintain a predetermined performance level even at the place where environmental temperature change is large, such as outdoors, which makes it very difficult to simplify a circuit configuration and to reduce the number of parts. Further, it is difficult to fully compensate the above characteristics change using the electrical circuit since the characteristics change itself is non-linear. Therefore, the angular rate sensor using the same oscillator cannot be applied to the model having narrow tolerance ranges in characteristics change with respect to environmental change.
In the angular rate sensor for stabilizing an image, which is often mounted in devices provided with an image output unit, such as a video camera, it is necessary to control oscillation frequency of the oscillator to fall within a predetermined range in order to prevent interference of electromagnetic wave traveling in the case of the device, vibration or the like, or mutual interaction between them. Further, in order to allow detuning degree, which shows a strong correlation with sensitivity or frequency responsibility, to have a target value, vertical oscillation frequency and lateral oscillation frequency must be separately adjusted. However, when an initial state value is very far from the target value, not only it becomes difficult to perform adjustment itself, but there is a possibility that the adjustment cannot be achieved.
Note that the detuning degree is a difference between resonance frequencies in the vertical and lateral directions. The smaller the detuning degree, the higher the sensitivity becomes.